CN115136322A - MOSFET with saturation contact and method for forming MOSFET with saturation contact - Google Patents

MOSFET with saturation contact and method for forming MOSFET with saturation contact Download PDF

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CN115136322A
CN115136322A CN202180015769.9A CN202180015769A CN115136322A CN 115136322 A CN115136322 A CN 115136322A CN 202180015769 A CN202180015769 A CN 202180015769A CN 115136322 A CN115136322 A CN 115136322A
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mosfet
contact
doped
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layer
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J·巴林豪斯
D·克雷布斯
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Robert Bosch GmbH
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Robert Bosch GmbH
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Abstract

A MOSFET having a saturated contact (200) is provided. A MOSFET having a saturation contact (200) has: an n-doped source region (16); a source contact (22); a contact structure (20) extending from the source contact (22) to the n-doped source region (16), forming a first conductive connection with the source contact (22) and a second conductive connection with the n-doped source region (16); a barrier layer (32); and an insulating layer (18); wherein the contact structure (20) has a section between the first and second conductive connection, which section is embedded between the barrier layer (32) and the dielectric layer (18) and is configured in such a way that a two-dimensional electron gas is formed in the section.

Description

MOSFET with saturation contact and method for forming MOSFET with saturation contact
Technical Field
The present invention relates to a MOSFET having a saturation contact and a method for forming a MOSFET having a saturation contact.
Background
In order to apply semiconductors having a wide band gap, such as silicon carbide (SiC) or gallium nitride (GaN), in power electronic devices, power mosfets (tmosfet) having a vertical channel region or power mosfets (vdmosfets) having a lateral channel region are typically used. The design parameters of the channel region make it possible in particular to set the on-voltage, the resistance in the on-state (on-resistance R) ON ) And saturation current (short circuit intensity). The channel region of such power MOSFETs is often additionally combined with another doped region forming a jfet (jmosfet) in order to produce a better shielding and thus a higher breakdown voltage. In the case of SiC, nickel alloyed to form nickel silicide is typically used as the source contact.
In a VDMOSFET, TMOSFET or JMOSFET according to the prior art, R is as low as possible ON And as low a saturation current as possible may be contradictory as an optimization objective. Low on-resistance R ON As a rule, the saturation current is correspondingly large and as a result the short-circuit strength is impaired. Accordingly, it may be desirable to maintain a sufficiently low R ON And a sufficiently low saturation current.
Disclosure of Invention
In various embodiments, a MOSFET is provided having a contact structure (also referred to as a saturation contact because the contact structure exhibits saturation characteristics at high voltages) that may have a significantly lower resistance at low voltages than in the channel of the MOSFET and a significantly higher resistance at high voltages than the channel resistance.
Thus, these two optimization objectives can be decoupled from each other, since the channel can now be optimized to achieve as low R as possible ON And the saturated contact may be used to limit current at high voltages.
In various embodiments, to produce such saturated contact characteristics in a contact structure, graphene may be used to contact a contact structure of a semiconductor material (e.g., SiC) of a MOSFET. Due to the high mobility of the two-dimensional electron gas (2DEG) in graphene, very low resistance can be achieved at small voltages. However, since the drift velocity of charge carriers (electrons) in a two-dimensional electron gas has a very low saturation voltage, their mobility drops sharply from the critical voltage. Thus, at high voltages, the current through a MOSFET (e.g., a power MOSFET) may be effectively limited by the resistance of the contact structure.
Fig. 1 shows the simulated output characteristic of a (power) MOSFET with a graphene contact structure connecting the source contact with the channel of the MOSFET (solid line) compared to a conventional TMOSFET (dashed line) and a pure graphene resistor (dotted line). At low voltages, the current is limited by the resistance of the channel of the MOSFET, while at high voltages, the contact resistance determines the current limit.
In this context, "low voltage", "small voltage" or "weak voltage" is to be understood as the following voltages: this voltage is in the order of magnitude of the operating voltage of the MOSFET, e.g. a few volts, e.g. up to about 10V. "high voltage" or "large voltage" is understood to mean the following voltages: the voltage is many times, such as two or more times, for example about 20V or more, the operating voltage.
One advantage of a MOSFET with a saturating contact is that the optimum parameters of reactive saturation current (and thus short circuit strength) and R in a MOSFET without a saturating contact ON Independently of each other can be optimized or optimized. Thus, a low R can be achieved while achieving a high short-circuit strength (small saturation current) ON 。
Drawings
Further developments of the aspects are set out in the dependent claims and in the description. Embodiments of the invention are illustrated in the drawings and are further explained in the following description. The figures show:
fig. 1 shows simulated transmission characteristic curves of MOSFETs with graphene contact structures according to various embodiments, compared to a state of the art TMOSFET and pure graphene resistance;
fig. 2 shows a schematic cross-sectional view of a MOSFET with a saturation contact according to various embodiments;
fig. 3 shows an equivalent circuit diagram of a MOSFET with a saturated contact according to various embodiments;
fig. 4A and 4B respectively show schematic cross-sectional views of a MOSFET having a saturation contact according to different embodiments;
fig. 5A, 5B and 5C respectively show schematic diagrams of methods for forming a MOSFET with a saturation contact according to different embodiments; and
fig. 6 shows a flow diagram of a method for forming a MOSFET with a saturated contact, in accordance with various embodiments.
Detailed Description
Fig. 2, 4A and 4B respectively show schematic cross-sectional views of a MOSFET with a saturation contact 200 according to different embodiments. Even though a JMOSFET having a vertical channel region is described below for illustration, it is understood that embodiments are also directed to MOSFETs having other structures, such as MOSFETs (tmosfet) having a vertical channel region without forming a JMOSFET, or such as MOSFETs (vdmosfets) having a lateral channel region.
A MOSFET having a saturation contact 200 may have an n-doped source region 16, a source contact 22, a contact structure 20 extending from the source contact 16 to the n-doped source region 16, forming a first conductive connection with the source contact 22 and forming a second conductive connection with the n-doped source region 16, a barrier layer 32 and a dielectric layer 18.
The MOSFET with the saturation contact 200 may also have further structures, such as a p-doped channel region 14, an n-doped drift region 12, a substrate 10, a drain connection 16, a gate region 28 and a gate dielectric 30, which may be formed in a manner substantially conventional for MOSFETs.
The MOSFET with the saturation contact 200 may have, for example, silicon carbide and/or gallium nitride and/or other (for example for power MOSFETs) semiconductor materials as the substrate 10 (which are doped or have been doped in order to form, for example, the n-doped source region 16, the p-doped channel region 14 and the n-doped drift region 12).
In various embodiments, the doping concentration in the n-doped source region 16 may be higher than the doping concentration in the n-doped drift region 12. For example, the source region 16 may be doped to approximately 1E19/cm 3 And drift region 12 may be doped to approximately 1E16/cm 3 . For simplicity, the term "n-doped" is used below.
The contact structure 20 can have a section between the first conductive connection and the second conductive connection, which is embedded between the barrier layer 32 and the dielectric layer 18 and is configured in such a way that a two-dimensional electron gas is formed or can be formed in the section.
The configuration of the two-dimensional electron gas 2DEG may result in a section of the contact structure 20 having a voltage-dependent resistance. For example, when the operating voltage of the MOSFET (or a voltage that is approximately as high as the operating voltage, e.g., between about 0V and about 10V) is applied, the voltage-dependent resistance may be small, e.g., less than the resistance of the p-doped channel region 14. When a voltage higher than the operating voltage is applied (e.g., several times or several times the operating voltage), the voltage-dependent resistance may be high, e.g., higher than the resistance of the p-doped channel region 14.
In various embodiments, the contact structure 20 may have a graphene layer, a layer system with at least one gallium nitride layer and at least one aluminum gallium nitride layer, a molybdenum disulfide layer or another layer system, which is adapted to form a two-dimensional electron gas. The layers or the layer structure can be formed, for example, in terms of layer thickness, number and relative position of the individual layers of the layer system, such that the formation of the 2DEG is achieved. The configuration parameters that should be considered to enable the construction of a 2DEG may be known or substantially known to those skilled in the art.
The contact structure 20 may extend completely (as exemplarily represented in fig. 2, 4A, 4B and 5A to 5C) over the n-doped source region 16 or (not represented) cover only a portion of the surface of the n-doped source region 16.
The contact structure 20 may extend completely (as exemplarily present in fig. 2, 4A, 4B, and 5A-5C) under the source contact 22, e.g., extend under only a portion of the source contact 22 (not present), and/or have an opening in a face thereof (fig. 4B), for example.
In various embodiments, the MOSFET with the saturation contact 200 may also have a p-doped shielding region 24, which may be arranged below the contact structure 20 adjacent to the n-doped source region 16. In this case, a section of the contact structure 20 may be located above the p-doped shielding region 24.
In various embodiments, for example where the contact structure 20 has graphene, the n-doped source region 16 has n-doped silicon carbide, and the p-doped shield region 24 has p-doped silicon carbide, the graphene may form a low resistance contact with the n-doped silicon carbide. The conductivity of the contact between the contact structure 20 and the p-doped screening region 24 may inherently be so poor that the barrier layer 32 is formed without further measures in the p-doped screening region 24, for example in a face adjoining the surface of the p-doped screening region 24. This is shown in fig. 2. In other words, the barrier layer 32 may be part of the p-doped screening region 24.
In the region where the contact structure 20 (e.g. graphene or if necessary another material forming an inherently poor contact with the p-doped shield region 24) is in contact with the source contact 22, in different embodiments a metal contact 42 may be formed between the contact structure 20 and the p-doped shield region, which also forms a good contact (i.e. has a low resistance) with the p-doped SiC. The metal contact 42 may have, for example, nickel, titanium, aluminum, or a compound thereof. For example, the nickel contact may be alloyed such that nickel silicide is formed. The p-doped screening regions 24 can be connected at the same time by means of metal contacts 42.
As the barrier layer 32 is formed at the interface between the graphene 20 and the p-doped shielding region (e.g., p-SiC region) 24, a two-dimensional electron gas is formed in the graphene layer 20 over the p-doped shielding region 24. The two-dimensional electron gas behaves almost as an ideal saturated contact. The mobility of the 2DEG is several orders of magnitude higher at small voltages than in SiC, so that only R is given at low voltages ON Adding negligible resistance. For this purpose, reference is also made to the equivalent circuit diagram in fig. 3. At higher voltages, the graphene layer 20 immediately enters saturation and only allows a constant current density over a wide voltage range (see fig. 1). Such a current density may be higher than the current density at which the MOSFET's operating point is located, but well below its saturation current density. In the linear range of the output characteristic curve of a MOSFET with a saturation contact 200, the current flow through the graphene 20 is therefore not limited, and conversely in the saturation region of a MOSFET with a saturation contact 200. As a result, the total current is effectively limited in the event of a short circuit and the short-circuit strength is therefore improved without the conduction characteristic of the MOSFET 200 with saturated contacts being impaired (durchlasseigenschafen).
In other words, the MOSFET has a strongly voltage-dependent resistance between the source region 16/contact structure 20-contact and the source contact 22/contact structure 20-contact as a "saturation contact". At low voltages, the resistance is ideally very small, and at high voltages, the resistance is significantly greater than the channel resistance (i.e. the resistance in the p-doped channel region 14) or the resistance of the drift region 12 of the MOSFET.
In various embodiments, the barrier layer 32 may be formed as a separate barrier layer 32, such as an oxide layer or a nitride layer, for example, where the contact structure 20 has a material or combination of materials that can form a good conductive contact with both the n-doped source region 16 and the p-doped shield region 24. This is exemplarily presented in fig. 4A and 4B.
As long as the barrier layer (different from that presented in fig. 4A and 4B) extends below the source contact 22, this barrier layer 32 may, if necessary, be opened at various locations (e.g. at regular intervals perpendicular to the drawing plane, i.e. e.g. along trenches in a gate formation in a TMOSFET) in order to also contact the p-doped screening region 24.
Fig. 5A, 5B and 5C respectively show schematic diagrams of methods for forming a MOSFET with a saturation contact 200 according to different embodiments.
Fig. 5A illustrates formation of a TMOSFET with a saturated contact 200 using a graphene saturated contact 20. After implantation of, for example, the p-doped channel region 14, the n-doped source region 16 and the p-doped shield region 24 and after activation of the implant (diagram a), the graphene may be grown, for example, globally (diagram b), at about 1700 ℃. Thereafter, trenches 50 may be formed and post-processed (e.g., chamfered) (at about 1400 deg.C; diagram c). All other processes, such as deposition of a gate dielectric 30 (e.g., gate oxide), annealing, deposition of polysilicon as the gate electrode 28, etc., may also be performed thereafter (diagram d) and may be limited to temperatures up to 1400 ℃.
Alternatively, as presented in fig. 5B, the trench 50 may be first formed and post-processed (e.g., chamfered) (diagram a) and thereafter the growth of the graphene 20 (diagram B) is performed. In a subsequent process, the graphene 20 in the trench 50 has to be locally removed again in this case (illustration c). Additional processes may be implemented as set forth in fig. 5A (diagram d).
In another variation illustrated in fig. 5C, the trench 50 is filled with a Carbon cap 52, a so-called "Carbon Capping" (diagram a), after its formation and post-processing. The carbon cap 52 may then be etched back so that it remains only in the trench 50 (shown as b). Thereafter, the graphene 20 grows. The trenches 50 filled with carbon cover 52 remain free of graphene 20 here (illustration c). Finally, the carbon cover 52 is removed by plasma etching. It is ensured here that the etching used removes only the carbon covering 52, but leaves the graphene 20 intact. For this purpose, for example, an oxygen plasma (shown as d) can be used.
Fig. 6 illustrates a flow diagram of a method 600 for forming a MOSFET having a saturated contact, in accordance with various embodiments. The method may have: forming an n-doped source region (in 610); forming a barrier layer (in 620); forming a contact structure in conductive contact with the n-doped source region, the contact structure extending laterally at least over a portion of the n-doped source region and over a portion of the barrier layer (in 630); and forming a dielectric layer over a section of the contact structure arranged above the barrier layer, wherein the contact structure is configured in the section such that a two-dimensional electron gas is formed in the section (in 640).

Claims (10)

1. A MOSFET (200) having:
an n-doped source region (16);
a source contact (22);
a contact structure (20) extending from the source contact (22) to the n-doped source region (16), forming a first conductive connection with the source contact (22) and a second conductive connection with the n-doped source region (16);
a barrier layer (32); and
an insulating layer (18);
wherein the contact structure (20) has the following section between the first conductive connection and the second conductive connection: the segment is embedded between the barrier layer (32) and the dielectric layer (18) and is configured such that a two-dimensional electron gas is formed in the segment.
2. The MOSFET (200) of claim 1,
wherein the segment has a voltage dependent resistance,
wherein the voltage dependent resistance is less than the sum of all other resistances of the MOSFET when an operating voltage of the MOSFET is applied, and
wherein the voltage dependent resistance is higher than at least one resistive component of the MOSFET when a voltage higher than the operating voltage, optionally several times the operating voltage, is applied.
3. The MOSFET (200) of claim 1 or 2,
wherein the contact structure (20) has one of a group of configurations, the group having:
a graphene layer;
a layer system having at least one gallium nitride layer and at least one aluminum gallium nitride layer; and
a molybdenum disulfide layer.
4. The MOSFET (200) of any one of claims 1-3, further having:
a p-doped shielding region (24) arranged below the contact structure (20) adjacent to the n-doped source region (16),
wherein the barrier layer (32) is part of the p-doped screening region (24).
5. The MOSFET (200) of claim 4, further having:
a metal contact (42) which at least partially electrically conductively connects the source contact (22) and the p-doped screening region (24),
wherein the metal contact (42) optionally comprises nickel, titanium, aluminum or a compound thereof.
6. The MOSFET (200) of any one of claims 1 to 3,
wherein the barrier layer (32) has a dielectric layer, such as an oxide or a nitride.
7. The MOSFET (200) of claim 6, further having:
a p-doped shield region (24) arranged below the barrier layer (32) adjacent to the n-doped source region (16),
wherein the barrier layer (32) has at least one opening, through which the source contact (22) and the p-doped screening region (24) are conductively connected.
8. The MOSFET (200) of any one of claims 1 to 7,
wherein the n-doped source region (16) has silicon carbide and/or gallium nitride.
9. The MOSFET (200) according to any of claims 1-8, further having:
a channel region;
wherein the channel region is formed laterally or vertically.
10. A method for forming a MOSFET, the method having:
forming an n-doped source region (610);
forming a barrier layer (620);
forming a contact structure in conductive contact with the n-doped source region, the contact structure extending laterally at least over a portion of the n-doped source region and over a portion of the barrier layer (630);
forming a dielectric layer (640) over a section of the contact structure disposed over the barrier layer;
wherein the contact structure is configured in the section such that a two-dimensional electron gas (650) is formed in the section.
CN202180015769.9A 2020-02-19 2021-02-15 MOSFET with saturation contact and method for forming MOSFET with saturation contact Pending CN115136322A (en)

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